BEIRUT 
Syrian rebels captured a government checkpoint on the main road to the country's second-largest airport Monday as opposition fighters pressed their campaign to capture the strategic facility in the northern city of Aleppo, activists said.

The rebels launched a major attack on the airport and the adjacent Nairab military airfield last week, and have since overrun the main army base protecting the area. Control of the airport would provide a significant boost for the opposition, and mark a strategic shift in the country's 23-month civil war.

The fighting has been raging for weeks, but it was only in early February after the rebels captured the strategic Aleppo neighborhood of Sheik Said near the airport, which has been shut down because of the violence, that the offensive gained momentum.

The Britain-based Syrian Observatory for Human Rights and the Aleppo Media Center reported that rebels captured the main fuel station on the Nairab base. The Observatory said there were several regime casualties and five rebels, including foreign fighters, killed before rebels withdrew from the station a few hours later.

The fight for the international airport in Aleppo, Syria's largest city and commercial hub, highlights the importance that both sides in the conflict place on controlling strategic assets that could provide an edge in the larger fight for the country.

Last week, rebels seized a hydroelectric dam and a major oil field, cutting off President Bashar Assad's regime from key resources necessary for its long-term survival. Rebels also seized an air defense base and fought near two other army installations in Syria's north.

Aleppo's airport is crucial in the broader fight for the city itself. Rebels launched an offensive on Aleppo in July, and quickly seized several neighborhoods. Since then, the fighting has settled into a bloody stalemate that has destroyed entire districts, killed thousands and forced thousands more to flee their homes.

Determined to hang onto the city, Assad's regime has thrown troops and resources into the fight. But his army has faced difficulties in sending reinforcements to Aleppo since October, when rebels captured the strategic town of Maaret al-Numan, about 80 kilometers (50 miles) southwest of Aleppo. Maaret al-Numan is on the highway that links Damascus with Aleppo.

The regime has not been able to fly supplies since the Aleppo airport closed weeks ago due to the fighting.

Rami Abdul-Rahman, who heads the Observatory, said over the past two weeks large army reinforcements were able to reach the town of Safira, near the city of Aleppo, after securing a desert road that links the central province of Hama with northern Syria.

He said the reinforcements, including dozens of vehicles and thousands of troops, have not been able to reach Aleppo because of heavy fighting in the town of Tal Aran between the city's international airport and Safira.

Safira is home to military production facilities that rebels have been trying to capture in recent past weeks with no luck.

"The regime is doing all it can to open a road to Aleppo," Abdul-Rahman said.